Current methods of producing virgin pulp utilize large manufacturing facilities (pulp mills) that produce pulp for multiple end users. Pulp mills process pulp from softwood, hardwood logs or chips or non-wood sources into fibers which are used in the manufacture of paper. Methods of processing virgin pulp to remove lignin to varying degrees range from purely mechanical methods such as grinding logs and wood chips into individual fibers, to chemical treatments of the tree chips or non-wood fibrous materials. These methods result in pulp fibers with different optical, physical, and archival properties.
Virgin pulp produced in the pulp mill is commonly used in one of three ways: 1) within a short time frame following production, in a liquid stock form; 2) in wet lap form in which the liquid pulp is dewatered to approximately 45-50% solids for later use; or 3) in a dry lap form, which is dried pulp having approximately 90-95% solids. Pulp used within a short time (i.e., (1) or dewatered (i.e., (2)) are collectively, “never dried”. The dry lap form and also wet lap form may be traded and transported for use in making paper at a later time, which may be from one to three days after pulping or after transport, sale, or storage for a period of three months or more. The majority of commercial market bleached pulp is made into dry lap or once-dried pulp, while recycled, market deinked pulp (MDIP) mills commonly produce recycled pulp in either wet lap form or fully dried form. After the fully dried or wet lapped pulp is shipped to a paper, tissue, or paper board mill, it is re-pulped with water and made into paper products. The quality of pulp or fibers deteriorates during the wet-lapping, drying or repulping processes. One way in which the pulp deteriorates is known as hornification, which includes reduced drainage, lowered water absorbance, decreased strength, stiffer fibers and greater fines content.
The time between pulp manufacture and re-pulping of the once-dried market virgin pulp varies greatly (e.g., from only a few days to potentially several years) while the typical time for wet lapped market pulps is from a few days to six months due to higher chances of mold growth and spoilage due to the higher moisture content. Any grade of paper may be produced from the market virgin or recycled pulps. Examples include fine printing and writing papers, cardboard, linerboard, corrugated paperboard, corrugated containers, boxes, tissue and towels. Each of these grades requires the pulp to have certain physical and chemical properties as well as operational properties of the stock such as good drainage. An important physical property is paper strength. The physical properties of never-dried or never-thickened (i.e., never wet lapped), wet-lapped, and fully-dried pulps differ greatly depending on type of fibers. Normally, the thickening or wet-lapping process and drying process hornify and crush the fibers, and materially worsen the strength properties. The never-dried or never-thickened pulp provides the greatest strength and drainage properties followed by wet-lapped pulp with the fully-dried pulp having the lowest measures of tensile and burst strength for each pulp fiber type and worst pulp drainage properties. The drying process further impairs sheet bulk.
Paper mills employ physical and chemical methods to provide the necessary physical and operational properties required to optimize the production and economic parameters of paper manufacture. The point/time of application of these drainage and strength treatments (i.e., shortly before application of the pulp into the paper machine) is important because it preserves fiber physical properties and may employ strategies such as charge deployment for which timing is critical.
One example of a physical strength enhancement approach is mechanical refining. Mechanical refining is conducted by applying energy to drive metal plates or other metallic shapes with extremely small inter-plate tolerances such that fibers are fibrillated as they are pushed through the plates. Fibrillating the fibers is especially important in the preparation of virgin fibers that are otherwise too “stick-like” and lack the necessary micro fibrils to form the key physical entanglements, greater inter-fiber surface area, and hydrogen bonding sites that provide the important strength aspects of sheets. Micro fibrils created from refining can be damaged through time, especially in the pressing and drying process. Consequently, most printing and writing and tissue/towel manufacturers that need further strength development, perform the mechanical refining to virgin and recycled pulps, and perform the mechanical refining of the pulps immediately before introduction into the paper machine. The major objective is to increase the fibrillation amount on fibers for better strength and at the same time to maintain a certain level of drainage, since refining hurts the drainage. For example, fibrils “catch” water and make it move more slowly through the sheet. Further, fibers are cut and shortened following mechanical refining.
Chemicals used to treat pulp in order to create the necessary physical and operational properties include synthetic compounds, naturally occurring compounds, and enzyme treatment. Because of the mechanisms of these chemicals, they are applied to the pulp, at most, shortly before the pulp is introduced into the machine chest or head box. Many synthetic strength aids involve charged materials, for example, anionic and cationic polyacrylamide treatments, which function to bridge fibers in order to hold them together to increase strength, drainage or retention. Time-sensitivity of application relative to the paper machine head box position is central to efficacy of polymeric treatments because electrical charge (present due to the charged materials) can be a fleeting, temporary effect. Charge chemistries are also weak and require a particular balance in application, so having too much time between chemical treatment and the actual formation of the sheet can be detrimental to strength development, drainage or retention. In most cases, the application of charge-based polymeric dry strength chemicals has been within or between a small number of process steps before the paper machine head box. These application points can include the inlet to the fan pump (the pump that delivers stock to the headbox), the machine chest (chest feeding the machine), the blend chest (before the machine chest), the final stock chests which feed into the blend chest (e.g., short and long fiber chests mixing into the blend chest), the repulper (the first step in taking fully dried or wet lapped pulp to low consistency), spraying chemicals on the forming wire or between sheet plies or onto the formed sheet or in a size press, or even in the machine, for example, spraying between sheet plies.
An example of a natural material which serves as a strength aid is starch. Starch and tree gums have a “glue-like” effect on the fibers. As with synthetic polymeric treatments, effective treatment with starch is dependent on the application site. Starch is typically applied to pulp shortly before the pulp is introduced into the paper machine either in the machine chest or the blend chest. However, it can also be applied on the machine after the pulp has undergone some process steps. For example, starch treatment can be employed by spraying starch on the sheet or between plies of multi-ply sheets for better bonding strength. In other applications, the starch strength treatment occurs after the sheet is fully formed and dried as the sheet passes through a vat to re-wet the sheet with a high concentration starch solution (e.g., in a size press).
The use of enzymatic treatments in the paper mill for the purposes of strength and drainage improvement has been investigated. Enzymes have been applied at or in the production line, shortly before introducing the pulp into the paper machine at similar points in the process to where mechanical and pre-machine chemical treatments can be applied. Examples are the machine chest, the blend chest, the post-fractionation chests, and even in the repulper at higher consistency. U.S. Pat. No. 6,066,233 to Olsen, et al. discloses treating ink free recycled fibers with an enzyme mixture (cellulose and pectinase) to achieve better drainage and produce a paper product with no loss in brightness. The treated recycled pulp was formed into paper product directly, i.e., there was no dewatering or wet-lapping or drying step between the enzymatic treatment and paper making. U.S. Pat. No. 5,110,412 to Fuentes, et al. discloses treating pulp with an enzyme such as a cellulase, hemicellulase or mixtures thereof. The pulp is treated with enzyme at the paper mill and then quickly used on the paper machine. U.S. Pat. No. 6,808,595 to Burns, et al. discloses treating fibers immediately before introducing the fibers into the paper machine with a hydrolytic enzyme to form aldehyde groups, and further treating with the hardwood fibers with a cross-linking agent and/or starch that forms bonds with the aldehyde groups for better strength and low linting during the tissue production. See also, U.S. Pat. No. 6,635,146 to Lonsky, et al. which discloses treating paper making fibers with a cellulolytic enzyme from 5,000 to about 200,000 ECU per kg of fibers prior to forming a paper sheet out of the treated pulp directly. U.S. Pat. No. 5,507,914 to Sarkar, et al. discloses treating pulp with a cellulolytic enzyme, refining the treated pulp, and then treating with a cationic polymeric coagulant and an anionic polymer at the vertical tank of the papermaking process. U.S. Pat. No. 6,939,437 to Hill, et al. discloses treating pulp in a paper mill with at least one cellulolytic enzyme and at least one cationic polymer before the paper machine.
The methods described above apply the enzymatic treatments at a point prior to introducing the pulp onto the paper machine or at some process steps in the paper mill shortly before the manufacturing of the paper. Thus, the pulp end user has to apply these treatments just before or during the paper making process. With respect to the wet lap or fully dried pulp, strength treatments in or immediately prior to the paper machine may not adequately compensate for the hornification which has occurred during the wetlapping or drying process and the storage and transportation time. Thus, there is still a need for a method of treating pulp, or a method to treat wastepaper in the furnish collection or bailing process which is ultimately shipped to the recycling mill, to decrease the effect of hornification and storage time on pulp physical properties during wetlapping, drying, storage and transportation of the pulp.
It is therefore an object of this invention to provide a method for reducing the effects of wetlapping, drying, and hornification or deterioration of the pulp following the wet-lapping or drying process.
It is also an object of this invention to provide wet lapped and fully dried pulp with improved drainage and fiber strength.